1. Field of the Invention
The present invention relates to a hydraulic transmission apparatus such as a torque converter and a hydraulic coupling which is used as a power transmission apparatus on vehicles and industrial machines, and more particularly to an improvement on a hydraulic transmission apparatus with a lockup clutch having a pump impeller driven by an engine, a turbine runner forming a circulation circuit between the pump impeller and the turbine runner and driving an output shaft, a side cover provided continuously to the pump impeller so as to form a clutch chamber communicating with an outer circumferential portion of the circulation circuit between an outer surface of the turbine runner and the side cover, a lockup clutch disposed in the clutch chamber so as to establish a direct connection between the side cover and the turbine runner, a clutch piston axially movably coupled to the turbine runner so as to divide the clutch chamber into inner fluid chamber of a turbine runner side and outer fluid chamber of a side cover side, a lockup control unit for generating a pressure difference between the inner fluid chamber and the outer fluid chamber so as to move the clutch piston forwards and backwards relative to an inner surface of the side cover using fluid supplied from an oil pump driven by the engine to the circulation circuit, a frictional engagement unit for establishing and cutting off an engagement between the clutch piston and the side cover in response to forward and backward movements of the clutch piston relative to the inner surface of the side cover and a resilient member disposed between the clutch piston and the turbine runner for biasing the clutch piston in a forward direction
2. Description of the Related Art
The hydraulic transmission apparatus with a lockup clutch has already been known as is disclosed in JP-A-11-63151 for example.
In the hydraulic transmission apparatus with a lockup clutch, the resilient member for biasing the clutch piston in the forward direction is intended to decrease a delay in which the clutch piston moves forwards when the lockup control unit is activated in order to put the lockup clutch in an engaged state to thereby improve the engaging responsiveness of the lockup clutch.
Incidentally, in the conventional hydraulic transmission apparatus with a lockup clutch, since the capacity of the oil pump driven by the engine becomes relatively low, when the engine is at idle, and moreover fluctuates, the receding or backward position of the clutch piston becomes inconstant, and consequently, the cut-off state of the frictional engagement unit becomes inconstant, resulting in a case where a generation of drag torque and a deterioration in fuel consumption may be called for. In addition, the inconstant backward position of the clutch piston means an inconstant forward stroke of the clutch piston, and this means, in turn, that the engaging responsiveness of the lockup clutch becomes inconstant depending on the running conditions of the engine.